Charles H. Spurgeon on Psalms 110:3

The subjects of the Priest King are willing soldiers. In accordance with the warlike tone of the whole Psalm, our text describes the subjects as an army. That military metaphor comes out more closely when we attach the true meaning of the words, "in the day of thy power." The word rendered, and rightly rendered, "power, "has the same ambiguity which that word has in the English of the date of our translation, and for a century later, as you may find in Shakespeare and Milton, who both used it in the sense of "army". Singularly enough we do not employ "powers" in that meaning, but we do another word which means the same thing —and talk of "forces", meaning thereby "troops" ..."The day of thy power" is not a mere synonym for "the time of thy might", but means specifically "the day of thine army", that is, "the day when thou dost muster thy forces and set them in array for the war". The King is going forth to conquest. But he goes not alone. Behind him come his faithful followers, all pressing on with willing hearts and high courage. Alexander McLaren, 1871.